The present invention relates to the preparation of water-soluble or water-swellable copolymers based on ammonium salts of acrylamidoalkylsulfonic acids and N-vinylcarboxamides, and to their use as thickeners, stabilizers of emulsions and dispersions, and as slip agents in cosmetic and pharmaceutical compositions for adjusting the viscosity of aqueous solutions.
Multicomponent systems which contain water or solvent, such as solutions, emulsions or suspensions, are frequently adjusted to higher viscosities or thickened for reasons of cost or performance or for stability reasons.
Thus, for example, by increasing the viscosity of the external or internal phase of emulsions or suspensions, it is possible to significantly prolong the time before the components of such a system separate, which becomes apparent from an extension in the shelf life. For many products, increasing the viscosity also improves their ability to be distributed uniformly, in particular on uneven surfaces. This is true particularly for skincare compositions and pharmaceutical ointments on the skin. In the case of many industrial products, such as wallpaper stripping agents, paint strippers or aircraft de-icers, the increased viscosity prevents premature run-off from the surface to be treated. The more uniform distribution and extended contact time result in an increase in effectiveness. As well as the performance advantages mentioned, the high viscosity of such preparations also offers further advantages during preparation, packaging, containerizing and storage, as well as during transportation, the thickening of acidic media being of particular significance from a safety viewpoint.
In general, the rheological properties during the preparation and/or formulation of cosmetic, pharmaceutical or technical preparations are a decisive criterion for the use of these products in practice. Even when used in very small quantities, the thickeners employed should lead to adequate thickening. However, the color and principal properties of the medium to be thickened must not be changed.
A large number of different systems are given in the specialist literature for adjusting the rheological properties of aqueous or solvent-containing systems, emulsions, suspensions. Known examples are cellulose ethers and other cellulose derivatives (e.g. carboxymethylcellulose, hydroxyethylcellulose), gelatin, starch and starch derivatives, sodium alginate, fatty acid polyethylene glycol ester, agar agar, tragacanth or dextrin. The synthetic polymers used are a variety of materials, such as, for example, polyvinyl alcohols, polyacrylamides, polyacrylic acid and various salts of polyacrylic acid, polyvinylpyrrolidone, polyvinyl methyl ether, polyethylene oxides, copolymers of maleic anhydride and vinyl methyl ether, and various mixtures and copolymers of the compounds given above.
However, said compounds exhibit various disadvantages upon use. Thus, for example, the cellulose derivatives and, generally, the materials based on natural raw materials and the formulations resulting therefrom are very susceptible to bacteria. In applications, they are mostly noticeable by the formation of unpleasant xe2x80x9cstringingxe2x80x9d gels. Fatty acid polyethylene glycol esters tend toward hydrolysis in the presence of water and the insoluble fatty acids which form cause undesired clouding. Thickeners of natural origin (e.g. agar agar or tragacanth) have a greatly fluctuating composition, depending on their origin.
EP-A-0 816 403 and WO 98/00 094 describe crosslinked homopolymers of 2-acrylamido-2-methylpropanesulfonates and their use as thickeners. EP-A-0 510 246 describes, inter alia, crosslinked copolymers of N-vinylcarboxamides and unsaturated alkylamides substituted by a sulfonate group, which are likewise suitable as thickeners. The sulfonic acid group here is exclusively in the form of the free acid or as alkali metal salt.
Surprisingly, we have now found that ammonium salts of various acrylamidoalkylsulfonic acids, in particular the ammonium salt of 2-acrylamido-2-methylpropanesulfonic acid, are sufficiently soluble in solvents which are acceptable for cosmetic applications, such as alcohols or alcohol mixtures, in particular in tert-butanol, and are therefore highly suitable for a copolymerization with N-vinylcarboxamides which are likewise soluble in these solvents or solvent mixtures, and as monomers which act as crosslinkers. In contrast to this, according to the prior art, it is obligatory to work in an aprotic solvent. Since the ammonium salt of 2-acrylamido-2-methylpropanesulfonic acid used for the polymerization is in ionic form, the crosslinked copolymer obtained no longer needs to be neutralized in an involved manner, but can be used as thickener immediately following polymerization and removal of the solvent. As a further advantage, the ratio of ionic to neutral building blocks can be controlled by virtue of the N-vinylcarboxamide incorporated as comonomer, thus enabling the thickening action and salt stability to be regulated and be better matched to specific requirements. The polymerization in alcohol or alcohol mixtures having a water content of less than 10% by weight and here in particular in tert-butanol furthermore gives products which, with regard to their residual content of solvent remaining in the product, are toxicologically acceptable and can therefore be used, for example, in cosmetic products.
The present invention provides water-soluble or water-swellable polymers, preferably having a particle size no greater than 10 xcexcm and which, in addition to from 0.01 to 5% by weight of crosslinking structures originating from monomers having at least two olefinic double bonds, comprise, in random distribution, from 1 to 50% by weight of the repeat structural unit (1) and from 49.99 to 98.99% by weight of the repeat structural unit (2), 
in which R1, R2 and R3 independently of one another are a hydrogen atom or a methyl group, and Z is a C1-C4-alkylene group.
Preferred polymers according to the invention comprise from 2 to 30% by weight of structural units of the formula (1), preferably derived from N-vinylformamide, from 69.5 to 97.5% by weight of structural units of the formula (2), preferably derived from the ammonium salt of 2-acrylamido-2-methylpropanesulfonic acid and from 0.5 to 2% by weight of crosslinking structures originating from monomers having at least two olefinic double bonds.
Crosslinkable structures originating from monomers having at least two olefinic double bonds are preferably derived from, for example, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane or other allyl or vinyl ethers, polyfunctional alcohols, tetraethylene glycol diacrylate, triallylamine, trimethylolpropane diallyl ether, methylenebisacrylamide or divinylbenzene.
Particularly preferably, the crosslinking structures are derived from monomers of the formula (3), 
in which R1 is hydrogen or methyl.
A preferred process for the preparation of the polymers according to the invention comprises, for the preparation of the polymers,
a) dissolving or dispersing from 49.99 to 98.99 parts by weight, preferably from 69.5 to 97.5 parts by weight, in particular from 84.5 to 96.5 parts by weight, of the ammonium salt of acrylamidoalkylsulfonic acid of the formula (4), 
xe2x80x83in which R3 and Z are as defined above, in an alcoholic solvent or solvent mixture, or dissolving or dispersing from 49.99 to 98.99 parts by weight of the free acrylamidosulfonic acid derived from (4) in an alcoholic solvent or solvent mixture and converting the acid into the ammonium salt by introducing ammonia or ammoniacal solution,
b) adding to the solution or dispersion obtained as in a) from 1 to 50 parts by weight, preferably from 2 to 30 parts by weight, in particular from 3 to 15 parts by weight, of the N-vinylcarboxamide of the formula (5) 
xe2x80x83in which R1 and R2 independently are as defined above,
c) adding to the solution or dispersion obtained as in b) from 0.01 to 5 parts by weight, preferably from 0.5 to 2 parts by weight, of one or more crosslinkers having at least two double bonds, and
d) starting the polymerization in a manner known per se using free-radical-forming compounds, and carrying out the polymerization at a temperature of from 10 to 150xc2x0 C., the alcoholic solvent or solvent mixture used in a) being chosen such that the resulting polymers are largely insoluble in the solvent or solvent mixture.
The polymerization reaction is preferably carried out in a water-soluble alcohol or a mixture of two or more alcohols having from 1 to 4 carbon atoms, preferably in tert-butanol. The water content of the alcohol or of the mixture of two or more alcohols must not exceed 10% by weight, since lumps can otherwise appear during the course of polymerization. Specifically, the type and quantity of solvent is chosen such that the quantity of ammonium salt of acrylamidoalkylsulfonic acid, in particular of 2-acrylamido-2-methylpropanesulfonic acid, used or generated by introducing ammonia or ammoniacal solution is largely dissolved or dispersed. xe2x80x9cLargely dissolved or dispersedxe2x80x9d means that even after the stirrer has been switched off, solid material does not settle out of the solution or dispersion. The polymer formed in the course of the reaction should, by contrast, be largely insoluble in the chosen solvent (or solvent mixture). xe2x80x9cLargely insolublexe2x80x9d here means that in the course of polymerization a readily stirrable pulpy polymer paste forms in which lumps or agglutination must not form. The filtrate which is obtainable by filtering the paste with suction must have a solids content of at most 5% by weight. If the polymers are soluble to a greater extent in the chosen solvent or solvent mixture, clumping can result during drying of the polymer paste.
The polymerization reaction itself is triggered in a manner known per se by free-radical-forming compounds, such as azo initiators (e.g. azobisisobutyronitrile), peroxides (e.g. dilauryl peroxide) or persulfates in a suitable temperature interval from 20xc2x0 C. to 120xc2x0 C., preferably between 40xc2x0 C. and 80xc2x0 C., and is continued over a period from 30 min to several hours.
The copolymer composition can be altered by varying the above-described feed ratio of the monomers, and the content of crosslinker and thus be used to achieve a tailored profile of properties. By incorporating more ammonium salts of acrylamidosulfonic acids, it is possible, for example, to improve the thickening action of the polymers, while by incorporating more % by weight of N-vinylcarboxamides, it is possible to improve the electrolyte compatibility of the polymers and their solubility in nonaqueous systems.
In contrast to polymers based on acrylic acid, which, in the neutral or slightly alkaline range in 1% strength aqueous solution, display viscosities of more than 30,000 mPa.s, but whose thickening ability (or the measured viscosity) deteriorates greatly with decreasing pH, the copolymers described according to the invention can maintain their viscosity (in 1% strength aqueous solution) up to an acidic pH of about 3.